1. Field of the Invention
The present invention relates to determining the locations of mobile stations, more particularly, to a method and system for determining the locations of mobile stations using directional corrections.
2. Description of Related Art
The Global Positioning System (GPS) is often used to determine the locations of mobile stations, for example, to provide location-based services. Such location-based services include services, such as roadside assistance, direction assistance, tracking, whether forecasts, etc., that make use of the locations of mobile stations. Although GPS-based positioning methods are often the most accurate type of positioning method, a mobile station may not always be able to receive good signals from a sufficient number of GPS satellites. This can occur, for example, because of poor weather conditions or because the mobile station is in an urban or indoor environment.
Because of the limitations of GPS-based positioning methods, terrestrial positioning methods have also been developed. In terrestrial positioning, signals from one or more terrestrial locations, such as signals transmitted by base station antennas in cellular wireless networks, are used to determine the location of a mobile station. One such terrestrial positioning approach that is used in CDMA networks is advanced forward link trilateration (AFLT).
In a typical AFLT approach, a mobile station measures the phases of the pilot signals transmitted from four base station antennas. Each pilot signal is synchronized to a standard time, such as a GPS time (although each base station antenna may have a particular time offset). As a result, the phase of a pilot signal from a base station antenna that is measured by a mobile station can be used to calculate the distance between the mobile station and the base station antenna. In this way, the distances between the mobile station and the four base station antennas may be calculated. These four distance measurements can then be used to calculate the mobile station's location and time offset, provided the locations of the base station antennas and their respective time offsets are known.
A mobile station's location may also be calculated using a hybrid approach. In a hybrid approach, measurements of one or more terrestrial signals are combined with measurements of signals from one or more GPS satellites to determine a mobile station's location. For example, one, two, or three of the distance measurements that would be used for AFLT may be replaced by measurements of signals from GPS satellites.
To support AFLT or hybrid approaches, CDMA networks often store the locations of base stations and their time offsets in a base station almanac (BSA). The base station locations may be stored in the BSA as latitude and longitude coordinates. The time offsets are typically stored in the BSA as forward link calibration (FLC) values. During a location determination session, the mobile station may measure the phases of the pilot signals from four base station antennas and report the phase measurements to the network. An element in the network, such as a position determining entity (PDE), may then calculate the mobile station's location based on the phase measurements from the mobile station and the base station locations and time offsets listed in the BSA.
However, the base station locations in the BSA may not be accurate for any number of reasons. For one thing, the locations may not have been collected with the level of precision required to provide good location fixes using AFLTS or hybrid approaches. Another problem is that the base station location listed in the BSA may correspond to the location of the transceiver, rather than to the location of the antenna. In some environments (such as urban areas), the antenna may be located an appreciable distance away from the antenna. Finally, the locations of antennas are often changed as wireless service providers expand and update their coverage. The changed antenna locations may not always be reflected in the BSA in a timely manner. All of these inaccuracies in the base station antenna locations can contribute to inaccuracies in mobile station locations that are calculated using AFLT or hybrid approaches.
Accordingly, there is a need to provide more accurate methods for determining the locations of mobile station based on terrestrial signal measurements.